As if they weren't already bright enough, the winter stars have drawn an ellipse around brilliant Jupiter, which is prominent in our evening sky until past midnight. (More precisely, of course, Jupiter has wandered into the familiar loop of bright winter stars.) Jupiter also finds himself due north of the brightest star in the sky, Sirius (the Dog Star) such that the two of them cross the local meridian at nearly the same time. As an example, the sky image above shows the evening of 17 February a few minutes before 9 p.m. At that time, Jupiter and Sirius both cross the local meridian of Madison. (They will do this as seen from other locations too, but the exact time will vary with distances east or west. The local meridian is the imaginary line that passes from north to south through the local zenith and divides the observer's sky in half.)
The other bright object within the “winter ellipse” is the star Betelgeuse—a red supergiant star vastly swollen in its advanced age. If placed at the center of our solar system, its outer edges would spill beyond the asteroid belt. As is typical of such massive stars, astronomers expect it to explode as a supernova any day now (meaning, in astronomical terms, in the next million years or so). Be sure to compare the distinct reddish cast of Betelgeuse with its neighbors to the south, Rigel, Sirius, and Procyon, which are all more whitish (hotter in surface temperature than Betelgeuse) to blue-white (much hotter). Rigel, like Betelgeuse, is part of the constellation Orion (look for the three obvious stars of his belt). Sirius (the brightest star in our sky, after the Sun of course) is in the constellation of Canis Major, the Great Dog. Sirius, as stars go, is not particularly brilliant in this company, but is prominent because it is nearby—if we could move the blue super-giant Rigel, for example, in to about the same distance as Sirius, Rigel would be about 5000 times brighter than Sirius! Neighboring Procyon marks the constellation Canis Minor, the Small Dog.
Arcing above Jupiter are three more whitish stars, Pollux and Castor (the twins of Gemini) and Capella, in the constellation Auriga, the Charioteer. Rounding out the ellipse on the west is the reddish star Aldebaran. An orange giant, though nowhere near as large as Betelgeuse, Aldebaran would still dwarf the Sun. In our sky, Aldebaran is the brightest star of Taurus, the Bull. And just to the west of Aldebaran, outside the ellipse, look for the little star cluster called the Pleiades.
If you are lucky enough to be stargazing on a clear, moonless night at a dark site relatively free of light pollution, you should have no problem finding the Milky Way. The winter ellipse of bright stars is crossed by the Milky Way running roughly from Sirius to between Aldebaran and Capella, and of course beyond. This is the disk of our own galaxy, seen from within and looking outwards away from the center into intergalactic space. To look inward, toward the center of our galaxy, you'll have to seek out Sagittarius, which we see in our evening skies, low in the south, in the summer.
Now that we have the big picture, so to speak, of the winter sky, look back at Jupiter. Notice anything unusual? Unless you have very exceptional eyes, you won't until you get out some binoculars or a small telescope. Even binoculars, if you can hold them steady on a cold night (or mount them on a tripod), will reveal some of Jupiter's bright companions: the Galilean moons. They might look like this. (see image) Galileo, of course, discovered them in 1610 with one of his primitive telescopes, which was big news in his day, and that's why we call them after him. They are fun to watch from day to day because you can see them change position as they orbit the giant planet. Galileo didn't realize right away that they were moons, but after a few night's observing, he figured it out. The news that there were moons orbiting something other than the Earth was sensational when Galileo published his discovery in 1610, and it made him world famous. He then went on to observe them carefully for quite a while until he had figured out their orbital periods.
Because they move rather quickly, the actual arrangement you will see depends on exactly when you look. In the image here they are shown, as you might see them in a small telescope, at just before 9 p.m. on 17 February, but they will shuffle the deck every day, so expect to see something different every time you look. That actually goes for star gazing in general: even from night to night you will see meteors, satellites, and our own rapidly changing moon, and that's with your eyes only. Even binoculars or a modest telescope will open up vistas that will take a lifetime to explore. Just begin observing carefully: turn those binoculars on the Pleiades, then explore the Milky Way, and keep looking up!
From the University of Wisconsin's "Space Place"
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